For decades, a sidewalk curb at Rose and Prospect streets in Hayward, Calif., tracked the slow creep of Earth along a notorious seismic fault line.

Then, last month, the sidewalk record was erased by a city crew, which replaced the curb with a wheelchair-accessible ramp.

The curb’s disappearance resulted in the loss of one of the most visible historical markers of the Hayward Fault, which runs 74 miles along San Francisco Bay, parallel to the San Andreas Fault. Over time, as the Pacific and North American tectonic plates slipped past each other along the fault, the curb slowly pulled apart.

The resulting offset was a perfect demonstration of the Hayward Fault’s steady movement of a few millimeters a year, said Luther Strayer, a structural geologist at California State University, East Bay. He brought his students on field trips to the curb every year.

The city had tried to fix the curb about a decade ago, Dr. Strayer said. “We wrote emails to City Hall, telling them that this is a world-class offset and asking them to please preserve it,” he said.

Back then, the city agreed not to touch it. This time, however, the geology community was not so lucky.

“It’s unfortunate that the curb got repaired,” said Kelly McAdoo, an assistant city manager in Hayward, “but obviously our City Council is really interested in pedestrian safety and wheelchair accessibility in our community.”

She noted that the city government has experienced a lot of turnover in the last decade, and that there was no documentation in the city’s computer system indicating that the curb should not have been altered.

Not all geologists are as dismayed as Dr. Strayer by the curb’s razing.